pylings with no workspace should auto-init instead of erroring

[abhiksark]

Part of the 0.3.0 adoption roadmap — First-run UX.

Why (adoption impact)

The #1 beginner trap. A new user installs pylings, runs pylings, and — if they're not inside an initialized workspace — gets a ManifestError (exit 2). They have no idea they were supposed to run pylings init --path ... first. We lose them at the very first command.

Current state

• pylings/cli.py main(): with no subcommand, args.command in (None, "watch", "start", "topics") calls run_tui(args.root, ...). args.root defaults to the current directory.
• If args.root has no info.toml, manifest loading raises ManifestError → caught at the bottom of main() → prints pylings: <error> and returns 2.
• Workspace creation lives in _cmd_init (pylings/core/...), invoked today only via the explicit init subcommand.

Scope / acceptance criteria

• When pylings runs with no workspace in args.root, do NOT hard-error. Instead either:
  • (preferred) detect "no info.toml here", print a one-line explanation, auto-init a workspace in a sensible default location (e.g. ./pylings-workspace or args.root), and launch into the first exercise; or
  • prompt the user No workspace found here. Create one in ./pylings-workspace? [Y/n] and proceed on yes.
• If a workspace already exists, behavior is unchanged.
• --root is still respected.
• Non-interactive/CI contexts (no TTY) fall back to a clear message + exit code, not a hang.
• Decide and document the default workspace path; keep it predictable.

Where to look

• pylings/cli.py (main, _cmd_init, _build_parser, the args.command in (None, ...) branch).
• pylings/core/manifest.py (ManifestError, load).

Out of scope

• Onboarding/tutorial overlay (separate issue Add pylings to the Rustlings third-party exercises list #10).

Verify

• In a fresh empty dir, pylings should get a beginner into exercise 1 (or a clear y/N prompt), not a traceback or bare ManifestError.
• Add/extend a CLI integration test under tests/integration/ for the "no workspace" path (assert it inits or prompts, doesn't exit 2 silently).

Difficulty

enhancement · good first issue — contained CLI change, ~half a day with tests.